Add const attribute to Schema

[kamilkloch]

Both OpenAPI 3.1 and AsyncApi 3.0.0 support const:

• OpenAPI 3.1 supports const, as it conforms to JSON Schema Specification Draft 2020-12: https://json-schema.org/specification-links#2020-12
• AsyncApi 3.0.0 Schema supports const: https://www.asyncapi.com/docs/reference/specification/v3.0.0

More importantly, for AsynApi 3.0.0, const looks like the only working mechanism to encode union types with a discriminator - see https://www.asyncapi.com/docs/reference/specification/v3.0.0#models-with-polymorphism-support. (Current AsyncApi schema for union types rendered by tapir is invalid: #3754)

[adamw]

There is const in the jsonschema that we use: https://github.com/softwaremill/sttp-apispec/blob/master/apispec-model/src/main/scala/sttp/apispec/Schema.scala#L55

I don't think we should be assing const to sttp.tapir.Schema. That's the high-level representaiton. Possibly, the translation between a tapir schema and an async api schema needs to be adjusted?

[adamw]

That's probably the place where the discriminators are generated:

tapir/docs/apispec-docs/src/main/scala/sttp/tapir/docs/apispec/schema/TSchemaToASchema.scala

Line 171 in 628ec8e

private def tDiscriminatorToADiscriminator(discriminator: TSchemaType.SDiscriminator): Discriminator = {

[adamw]

I wonder if the way discriminators are defined for OpenAPI and AsyncAPI diverges? But hopefully we can generate something that works for both cases

[kamilkloch]

There is const in the jsonschema that we use: https://github.com/softwaremill/sttp-apispec/blob/master/apispec-model/src/main/scala/sttp/apispec/Schema.scala#L55

True, I think it is not being set anywhere in the schema generation code, though.

I don't think we should be assing const to sttp.tapir.Schema. That's the high-level representaiton. Possibly, the translation between a tapir schema and an async api schema needs to be adjusted?

If I am not mistaken, currently OpenAPI and AsyncApi schemas are rendered in the same way, this leads to an illegal async api spec when using .withDiscriminator: #3754. So yes, it looks like a translation between a tapir schema and an async api schema needs to be adjusted.

[kamilkloch]

I wonder if the way discriminators are defined for OpenAPI and AsyncAPI diverges? But hopefully we can generate something that works for both cases

AsyncApi does not support mapping for the discriminator, also discriminator must be a String. It looks like separate rendering paths might be needed - OpenApi conforms to JSON Schema Specification Draft 2020-12, AsyncApi does not.

[adamw]

Ah I see. Then I guess we'll need a flag for the TSchemaToASchema class to make this configurable

[kamilkloch]

Ah I see. Then I guess we'll need a flag for the TSchemaToASchema class to make this configurable

What would be your suggestion for the flag? Discriminator-specific (we might stumble on other asyncapi/openapi incompatibilities in the future), or perhaps enum OpenApi/AsyncApi/JsonSchema? It seems that ASchema is only used for rendering to one of those specs.

[adamw]

Let's start with a simple Boolean flag if we want to generate mappings or consts for discriminators. If there are more such cases, we'll refactor. ASchema is an alias for the sttp-apispec Schema, I think it's used by both?

[kamilkloch]

OK, let's roll with a Boolean flag. Yes, ASchema is used by both.

[kamilkloch]

Closining for now, see #3765.